Iv] NATURE OF THE RADIATIONS 147 
study of the secondary radiations produced by radio-active sub- 
stances. In his earliest observations he noticed that radiographs 
of metallic objects were always surrounded by a diffuse border. 
This effect is due to the secondary rays set up by the incident 
rays at the surface of the screen. 
The secondary rays produced by the @ rays are very feeble. 
They are best shown by polonium, which gives out only a rays, 
when, in consequence, the results are not complicated by the 
action of the @ rays. Strong secondary rays are set up at the 
point of impact of the @ or cathodic rays. Becquerel found that 
the magnitude of this action depended greatly on the velocity 
of the rays. The rays of lowest velocity gave the most intense 
secondary action, while the penetrating rays gave, in comparison, 
scarcely any secondary effect. In consequence of the presence of 
this secondary radiation, the photographic impression of a screen 
pierced with holes is not clear and distinct. In each case there is 
a double photographic impression, due to the primary rays and the 
secondary rays set up by them. 
These secondary rays are deviable by a magnetic field, and in 
turn produce tertiary rays and so on. The secondary rays are in all 
cases more readily deviated and absorbed than the primary rays, 
from which they arise. The very penetrating y rays give rise to 
secondary rays, which cause intense action on the photographic 
plate. When some radium was placed in a cavity inside a deep 
lead block, rectangular in shape, besides the impression due to the 
direct rays through the lead, Becquerel observed that there was 
also a strong impression due to the secondary rays emitted from 
the surface of the lead. The action of these secondary rays on 
the plate is so strong that the effect on the plate is, in many cases, 
increased by adding a metal screen between the active material 
and the plate. 
The comparative photographic action of the primary and 
secondary rays cannot be taken as a relative measure of the 
intensity of their radiations. For example, only a small portion 
of the energy of the 8 rays is in general absorbed in the sensitive 
film. Since the secondary rays are far more easily absorbed than 
the primary rays, a far greater proportion of their energy is ex- 
pended in producing photographic action than in the case of the 
10—2 
